1. Field of the Invention
The invention relates to the construction and fabrication of thin, flexible resonant circuits with precise characteristics, particularly for use in electronic security and article theft detection systems.
2. The Prior Art
Article theft detection systems are known in which electromagnetic waves are generated at a checkpoint and are caused to sweep repetitively at a given rate through a predetermined frequency range. For example, as disclosed in U.S. Pat. No. 3,500,373, issued Mar. 10, 1970, a resonant electrical circuit tag is affixed to articles to be protected, the electrical circuit being resonant at a frequency within the swept frequency range. Changes in energy level which occur at the sweep repetition rate are detected, indicating presence of the tag in the field. The electrical circuit comprises a coil and a capacitor connected to form a resonant loop. The coil may comprise a number of windings of thin wire, each end of which is soldered or otherwise electrically connected to one terminal of a capacitor. A disadvantage of tags having a resonant circuit of this type is that significant hand labor, which is relatively costly, is required in their fabrication. A further disadvantage of such tags in their thickness, particularly in the region of the capacitor, which presents problems in some applications (such as in libraries, where the tags are affixed to book covers or pages, and in cases where the tag is laminated between sheets provided with adhesive so that the tag may be applied in the manner of an adhesive label).
U.S. Pat. Nos. 3,863,244 issued Jan. 28, 1975, and 3,967,161 issued June 29, 1976 disclose resonant tag circuits which are fabricated by printed or etched circuit techniques. The tag circuit comprises an insulative substrate having one portion of the circuit formed on one side of the substrate and another portion of the circuit formed on the opposite side of the substrate. Electrical connection is made between the portions of the circuit on opposite sides of the substrate by means of a conductive pin or eyelet extending through the substrate, or by means of a spot weld joining confronting circuit areas. U.S. Pat. No. 4,021,705, issued May 3, 1977, discloses a similar type of resonant tag circuit.
U.S. Pat. No. 3,913,219, issued Oct. 21, 1975, discloses a fabrication process for planar resonant tag circuits, in which both sides of a web of insulative material are provided with a conductive material to serve as conductive surfaces from which circuit patterns are formed by printed and etched circuit techniques. Electrical connection is established between the two conductive patterns on opposing faces of the web by welding confronting conductive surfaces, such as by ultrasonic welding or cold-welding with the aid of a tool having a chisel-like tip.
The techniques heretofore known for fabricating planar resonant tag circuits are not entirely satisfactory for volume production of low cost circuits with accurate tolerances and uniform electrical properties from tag to tag. Drawbacks of the known techniques which are utilized to chemically etch complete circuits on two faces of the insulative substrate web include the cost of the process involved, and the difficulty in maintaining close tolerances in the processes. Moreover, it is difficult to establish a reliable and durable electrical contact between the two opposing conductive surfaces. The cold-welding technique disclosed in U.S. Pat. No. 3,913,219, for example, results in low yields of usable tags, poor quality, and rejects in the field when the tag is bent.
The prior art process disclosed in U.S. Pat. No. 3,913,219 uses printed circuit etching techniques combined with printing of etch-resistant patterns on opposite surfaces of an insulative material of specific thickness and with specific dielectric properties. It is difficult with this technique to mass-produce resonant circuits for use at relatively higher frequencies within desired resonant frequency tolerances of 10% (plus or minus 5% of the center resonant frequency) due to variation in thickness and dielectric properties of the insulative material, variations of the printing and etching processes, and misalignment of the patterns on the opposing web faces.
Federal Communication Commission requirements dictate that the frequency and power level of the swept electromagnetic waves be held within fairly close tolerances, which in turn requires that the resonant tag circuits have a relatively high Q and that their resonant frequencies fall within a narrow range in order to assure that they will be detected by the system.